Acute myocardial infarction, and subsequent failure of the mammalian heart to restore lost cardiac tissue, is one of the leading causes of death worldwide. There is an urgent need to develop therapies that facilitate survival or regeneration of healthy heart tissue after ischemia, which would have an enormous social and economic impact worldwide. The zebrafish (Danio rerio) possesses the robust ability to regenerate functional cardiac tissue after multiple types of ventricular injury, including induction of cardiac cell death by cryoinjury. Significant progress has been made in understanding the process of heart regeneration in zebrafish. For example, following cardiac injury, remaining cardiomyocytes are thought to dedifferentiate, disassemble sarcomeric structures, and proliferate to replenish lost cardiomyocytes. However, it remains unclear how these remaining cardiomyocytes sense and respond to injury, including how transcription factor regulatory networks function to elicit sarcomere disassembly and cardiomyocyte dedifferentiation. The goal of this proposal is to investigate the transcription factor regulatory networks that function to promote heart regeneration in zebrafish. Transcription factors are known to regulate the chromatin landscape based on their ability to bind and recruit chromatin-modifying enzymes to target loci. Significant changes on the level of chromatin must likely take place in order for adult cardiomyocytes to respond to injury and elicit a regeneration response. My preliminary data indicate that Jun/Fos transcription factors respond to cardiac injury and orchestrate downstream changes in the cardiomyocyte chromatin landscape to promote cardiac regeneration. The experiments proposed here aim to further understand the link between the innate immune response to cardiac injury and activation of jun/fos expression in cardiomyocytes. Furthermore, I aim to determine the molecular mechanisms of Jun/Fos action during zebrafish heart regeneration through elucidation of direct downstream target loci and protein interaction partners that mediate changes in chromatin accessibility. Lastly, I will determine whether JUN/FOS transcription factors also play a similar role during cardioprotection and whether overexpression of Jun/Fos can stimulate regeneration in an adult mouse model. The results of the studies proposed here will provide deeper mechanistic insight into the genetic and epigenetic regulatory networks that function to promote heart regeneration. Understanding these molecular mechanisms has practical implications for guiding the development of therapies to promote regeneration in the adult mammalian heart following myocardial injury.